They did stack at some time but was fixed. Only armor debuff per mob.
Same as inspiration doesn't stack.
Hopefully I'm wording this clearly: your own applied debuffs don't stack, but you can have more than one per mob.
I can remember clearly seeing both mine and other gunslinger's armour debuffs on operation bosses. That is to say, you can't stack more than one of your OWN debuff, but one mob could have multiple debuffs from multiple players, even the same kind of debuff from the same class.
About it being fixed, the debuffs of some classes (Troopers, Guardians, probably others I've forgotten) were changed from stacks to consistent % for a set amount of time.